1. Field of the Invention
The present invention relates to a scanning optical system and an image forming apparatus using the same and, more particularly, to a scanning optical system suited to an apparatus, such as a laser beam printer (LBP) or digital copying machine, which makes the light beam emitted from a light source means strike a light deflector at a predetermined angle with respect to a plane perpendicular to the rotation axis of the light deflector, deflects the light beam, and scans it on a scanned surface, thereby recording image information.
2. Description of the Related Art Various scanning optical systems are disclosed in, for example, Japanese Patent Application Laid-Open Nos. 6-18800, 7-27991, and 9-230274, which make the light beam emitted from a light source means strike a polygon mirror as a light deflector at a predetermined angle with respect to a plane perpendicular to the rotation axis of the light deflector, deflect/reflect the light beam, and scan it on a scanned surface, thereby recording image information.
According to Japanese Patent Application Laid-Open No. 6-18800, in the scanning optical system which makes the light beam emitted from the light source means strike the polygon mirror at a predetermined angle with respect to a plane perpendicular to the rotation axis of the polygon mirror, deflectively reflects the light beam, and scans it on a scanned surface through a scanning lens, thereby recording image information, a post objective optical system is disclosed, in which a main scanning cross-section of a deflection surface (polygon surface) of the polygon mirror has an elliptic shape, and the scanning lens is displaced in the sub-scanning direction (a direction perpendicular to a main scanning plane which is a plane scanned by the light beam deflectively reflected by the polygon mirror).)
According to Japanese Patent Application Laid-Open No. 7-27991, in the scanning optical system which makes the light beam emitted from the light source means strike the polygon mirror at a predetermined angle with respect to a plane perpendicular to the rotation axis of the polygon mirror, deflects/reflects the light beam, and scans it on a scanned surface through a scanning lens, thereby recording image information, a post objective optical system is disclosed, in which a main scanning cross-section of the polygon mirror has an elliptic shape, and only the incident surface of the scanning lens is shaped to be displaced in the sub-scanning direction.
According to Japanese Patent Application Laid-Open No. 9-230274, in the scanning optical system which makes the light beam emitted from the light source means strike the polygon mirror at a predetermined angle with respect to a plane perpendicular to the rotation axis of the polygon mirror, deflectively reflects the light beam, and scans it on a scanned surface through a cylindrical lens or cylindrical mirror, thereby recording image information, an optical system is disclosed, in which the cylindrical lens or cylindrical mirror is positioned to make the light beam to be incident on the cylindrical lens or cylindrical mirror it at a predetermined height away from the generating-line thereof.
However, various problems are posed in the above-described conventional scanning optical systems, as described below.
The scanning optical system disclosed in Japanese Patent Application Laid-Open No. 6-18800 is the post objective optical system in which the light beam emitted from the light source strikes the polygon mirror at a predetermined angle with respect to a plane perpendicular to the rotation axis of the polygon mirror, and the light beam is incident on the polygon mirror from the center in the deflection angle of the polygon mirror in the main scanning direction. A main scanning cross-section of a deflection surface of the polygon mirror is formed into an elliptic shape, and the scanning lens is displaced in the sub-scanning direction, thereby correcting any deformation of the spot shape while correcting f.theta. characteristics and curvature of field.
In an optical system using such an incidence method, i.e., a so-called sub-scanning oblique incident optical system, the surface formed by the light beam deflectively reflected by the deflection surfaces does not become a plane but becomes a conical surface like the one shown in FIG. 11. When such a light beam strikes the scanning lens, the light beam incident positions shift from each other on the central portion and two end portions of the lens in the sub-scanning direction. In this reference, the resultant spot shape deformation is corrected by displacing the scanning lens in the sub-scanning direction from a plane which includes the light beam deflectively reflected by the polygon mirror and is parallel with the main scanning plane.
In this reference, however, since the scanning lens is relatively near the scanned surface and far from a deflection surface, the shifting amount of the light beam incident positions in the sub-scanning direction on the scanning lens surface becomes large. This makes it difficult to properly correct any deformation of the spot shape only by displacing the scanning lens in the sub-scanning direction.
In addition, since the scanning lens described in this reference has a relatively strong power in the sub-scanning direction, when the scanning lens is displaced in the sub-scanning direction, the spot shape deteriorates in the sub-scanning direction. That is, spot shape deformation is not satisfactorily corrected.
In the scanning optical system disclosed in Japanese Patent Application Laid-Open No. 7-27991, to solve the same problem as described above, only the incident surface of the scanning lens is shaped to be displaced in the sub-scanning direction. In this case, however, since aberrations other than f.theta. characteristics are properly corrected, scarifying the f.theta. characteristics, the f.theta. characteristics are corrected by continuously changing the oscillation timing of the semiconductor laser.
If, however, such a correction is simply performed, since the scanning speed on the scanned surface is not constant, the amount of light on the scanned surface varies. If the emission time is continuously changed as well as the oscillation timing to keep the amount of light uniform, the apparent spot diameter changes in the main scanning direction. This makes it difficult to obtain good optical performance.
According to Japanese Patent Application Laid-Open No. 9-230274, curvature of field is corrected by positioning the cylindrical lens or cylindrical mirror to make a light beam strike the cylindrical lens or cylindrical mirror at a predetermined height away from the generating-line thereof. However, no consideration is given to the above spot shape deformation. In addition, in a scanning optical system having an arrangement like the one disclosed in the reference, satisfactory optical performance cannot be obtained only by simply correcting curvature of field alone, because spot shape deformation like the one descried above occurs.